The invention relates to making carbon bodies from bodies made of carbon-precursor material.
A particular field of application of the invention lies in making carbon fiber fabrics or preforms for use in constituting fiber reinforcement for parts made of thermostructural composite material such as carbon/carbon (C/C) composite and carbon-reinforced ceramic matrix composite. Such carbon fiber preforms are conventionally obtained from carbon-precursor fiber fabrics which are better at withstanding the textile operations required for shaping said fabrics than are carbon fibers.
Amongst carbon precursors, in particular precursors in the form of fibers, preoxidized polyacrylonitrile (PAN) is commonly used. At least for certain uses, it is necessary not only to transform preoxidized PAN fiber fabric into carbon, but also to eliminate any metals or metallic impurities that come from the precursor, i.e. mainly sodium. That is why carbon bodies are usually made from preoxidized PAN carbon-precursor bodies in two successive stages:                a first stage of carbonization proper in which the carbon precursor is transformed chemically into carbon, with this being performed on an industrial scale in an oven by progressively raising the temperature to which the oven is heated to about 900° C.; and        a second stage of heat treatment at high temperature, this treatment likewise being performed in an oven, with the temperature being raised progressively to above 1000° C., generally to a temperature in the range 1400° C. to 1650° C., so as to eliminate sodium by sublimation, or indeed to very high temperature, up to 2000° C. or 2200° C., or even 2500° C. so as to confer particular properties on the carbon fibers and so as to eliminate any other metallic impurities.        
During the first stage, generally performed substantially at atmospheric pressure while sweeping with an inert gas such as nitrogen, transformation of the precursor is completed so as to achieve a carbon content greater than 95% and possibly as much as 99% or more. The loss of mass is large, being about 50%, and it is accompanied by the production of a large volume of gaseous effluent, which effluent contains in particular nitrites, specifically cyanides, which must be treated.
The second stage is performed under low pressure, likewise while being swept with an inert gas such as nitrogen or argon. The flow rates of the sweeping gas and of the gaseous effluent are much lower than during the first stage.
The two stages are implemented in appropriate different installations. On an industrial scale, each of these stages lasts for several days. The process of obtaining carbon bodies without sodium starting from preoxidized PAN carbon precursor is therefore lengthy and expensive.
The same problems are encountered with carbon-precursor fibers other than preoxidized PAN and likewise containing sodium or other metals such as magnesium or calcium that need to be eliminated, and also when it is necessary to eliminate metals or metallic impurities such as iron, nickel, or chromium, for example, which require heat treatment at high temperature, typically up to 2000° C. and more in order to enable them to be eliminated by evaporation.